A photostimulable plate, also referred to as Photostimulable Storage Phosphor (PSP), is to detect image data relating to an object/human body which is exposed to X-rays. The method of radiographic image acquisition using a PSP will be referred to as CR (for Computed Radiography) in the following.
The photostimulable plate or PSP comprises a substrate which serves as a support for a phosphor layer on top of which is coated a protective layer.
In particular, a CR system includes an X-rays controlled source which illuminates an object with X-rays and a photostimulable plate or PSP disposed behind the object and which acquires the X-rays attenuated by their passage through the object and transforms them into a latent image that is stored within the PSP's phosphor layer.
Such a CR system further includes a CR reader, the purpose of which is to extract the image data carried by the photostimulable plate or PSP.
The CR system then employs digital electronics in order to process the extracted image data and produce an image under the form of a computed radiograph.
In a known manner, a CR reader comprises a laser source which emits a laser beam focussed at a spot on the surface of the PSP.
The laser beam illuminates a surface spot and stimulates the phosphor layer according to the well known photostimulated luminescence principle.
The laser light constituting the beam is of red colour, for example.
Following this principle, a portion of incident red laser light is converted into stimulated blue light which, in turn, is received and detected by a Photomultiplier Tube (PMT). The intensity of the radiated blue light is proportional in particular to the latent image data. The PMT converts the intensity of the blue light into a digital signal which is then transformed into a radiological image. This image is intended to be exploited by the dentist and may be, for example, displayed for an easier exploitation.
In order to improve the collection of the stimulated blue light, an optical device such as a reflecting cavity is arranged in close relation to the surface portion, between the latter and the PMT.
It is to be noted that the remaining portion of the incident red laser light which is not converted into blue light scatters in different directions beyond the portion of the phosphor layer which is illuminated by the laser beam.
This is detrimental to the efficiency of the CR reader.
A part of the scattered red light is also directed toward the PMT, which requires the use of an interference filter between the PMT and the PSP. As the PMT cannot discriminate between signal data (blue light) and parasitic data (red light), the interference filter is used to filter red light out, with a rejection ratio of for example greater than 10 exp-6.
Further, once the reading is completed the CR reader is moved over a small distance relative to the PSP so that another surface portion thereof is illuminated and another image data reading is performed as briefly mentioned above.
These operations are repeated until the whole PSP is read.
A drawback of this reading method is that it takes several seconds to read the whole PSP. This duration depends on the size of the PSP and the number of the scanning steps.
Accordingly, it is an object of the invention to increase the efficiency when reading a PSP.
It is another object to reduce the time taken for reading a PSP.
It is yet another object to make easier the reading of a PSP.